Flash Flood and Flood in Alaska, August 14 to 16, 2023

Southcentral and Southwest Alaska Forecast Discussion...UPDATED
National Weather Service Anchorage AK
415 PM AKDT Mon Aug 14 2023

.ANALYSIS AND UPPER LEVELS...
The satellite loop looks a lot like it did this time yesterday
with the jet stream running south of the Aleutians to the Gulf
and a distinct low in the Gulf. Today's low is farther southeast
and elongating more than the one yesterday, but it did bring in
some gale force winds and has produced a few lightning strikes
over the Gulf throughout the day. Like yesterday, there is another
low developing along the jet stream south of the Aleutians and
heading toward the Gulf. The rest of Southwest and Southcentral
Alaska is under a broad upper level low and low level southerly
flow. This is keeping it moist with lots of lower level clouds
having trouble burning off during the day. The areas of greatest
instability this afternoon and evening are over the northern
Susitna Valley and also along the north Gulf coast east of
Middleton Island.

&&

.MODEL DISCUSSION...
Back to back low pressure systems are anticipated to impact much
of Southcentral AK and the central Aleutians through mid week.
Model guidance remains in good agreement. The first Low,
currently situated within the Western Gulf of Alaska, should
continue on its eastward trajectory and weaken by Tuesday
afternoon as it moves onshore. On its heels, another low pressure
system is expected to approach the Central Aleutians by late
tonight. There remains some disagreement on the trajectory of the
2nd Low, however incoming 12z models/ensembles are coming into
better agreement of a potentially deeper & stronger low pressure
system pushing into the Gulf of Alaska by Tuesday evening into
Wednesday.

&&

.AVIATION...
PANC...VFR will persist through much of the TAF period. ESE winds
out of Turnagain Arm may briefly impact ANC this late
afternoon/evening, but confidence remains low at this point as
the jet is already weakening. Otherwise, showers across the
Anchorage bowl will also diminish this evening, though can't rule
out a brief impact to ANC. An approaching low pressure late
tomorrow will bring increased cloud cover once again and a chance
for rain late Tuesday evening through Wednesday.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...

Scattered rain showers decrease overnight in the higher
elevations of the Talkeetnas and the Alaska Range. Rain showers
will linger along the coast as a low in the Gulf weakens. An area
of gale force winds may accompany this front, south of Cape
Suckling before midnight. A stronger system follows closely
behind Tuesday afternoon, with increasing winds and widespread
showers. There is the potential for isolated thunderstorms to
develop before noon over the Susitna Valley and the Talkeetna
Mountains, ahead of the main front. Daytime heating will be
competing with the incoming cloud shield associated with the
trailing front, reducing opportunities for convection to initiate.
Approximately one quarter to half an inch of storm total rainfall
may be possible for many inland locations, and up to one and a
half inches for coastal locations of eastern Prince William Sound
and Cordova by Wednesday afternoon. Precipitation clears Thursday
with a brief ridge.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA/BERING SEA/ALEUTIANS (Days
1 through 3: Tonight through Thursday)...

A broad upper-level low is centered just east of Atka this evening,
stretching its influence across the Bering Sea/Aleutians, with
weak troughing extending over Southwest Alaska, where the weak
cyclonic flow is helping to promote scattered showers. This
evening, some degree of clearing over the Western Alaska Range
should be able to allow enough surface heating to fuel isolated
thunderstorms. Shower activity will wane briefly tonight as
temperatures cool, before a potent shortwave drawn up from the
North Pacific takes aim at Southwest Alaska for Tuesday. The
shortwave will be accompanied by a weak, but relatively compact
surface low which will push its front across the Alaska Peninsula
tonight, bringing steady rain which will spread northward into
Bristol Bay by Tuesday morning. Expect temperatures to struggle to
break through the mid 50s for Bristol Bay, which will be the
wettest portion of the forecast area for Tuesday.

Ahead of the front, some instability may support another round of
thunderstorms Tuesday afternoon, depending how much clearing in
cloud cover allows for surface heating over the Middle Kuskokwim
Valley and the Kilbuck Mountains. However, the more convective
mode of precipitation will change over to steadier stratiform rain
during the course of the evening as more stable air works north.

The low center tracks quickly into the northern Gulf of Alaska by
Tuesday night, drawing gusty small craft advisory winds through
Bristol Bay and across the Alaska Peninsula into the North
Pacific. With increasing drier, westerly flow aloft, precipitation
will begin to decrease across Southwest Alaska overnight while a
surface ridge builds in from the west. By Wednesday, high pressure
across the Bering Sea suppresses most precipitation with the
exception of some upslope showers along the western slopes of
mountains in Southwest Alaska. Gusty westerly winds continue with
low pressure lingering in the Gulf. Winds diminish somewhat for
Thursday as the pressure gradients weaken, with continued drier
weather. The main exception looks to be the northern Bering Sea,
which will see southerly winds along the northern periphery of the
ridge drawn up by low pressure in the Chukchi Sea.

&&

.LONG TERM FORECAST (Days 4 through 7: Friday through Monday)...

Latest model guidance (12Z runs) has pretty much flipped for
Friday, showing high pressure ridging building over the southern
Alaska Peninsula Thursday night and moving into the Gulf of Alaska
Friday morning. Whereas older runs were hinting at a low moving
through. The bad news is that due to the very little consistency
and the chance they can always flip back if they decide to,
confidence remains low at this point. The relatively okay news is
they are all being inconsistent together. The good news is that
the low dropping south from Kamchatka towards the Western
Aleutians on Sunday is still a go.

&&

SYNOPSIS AND MODEL DISCUSSION...EZ
AVIATION...MJB
SOUTHCENTRAL ALASKA...ER
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CQ
LONG TERM...KM/EZ